Purpose: :
Hypoxia is a driving force for angiogenesis. The degree of hypoxiadetermines whether this angiogenesis is pathological or physiologicalbut as yet cannot be quantified. Insight into the pathogenesisof human ischaemic retinopathies can be gained with a mousemodel based on oxygen-induced retinopathy (OIR). In this model,7 day old (P7) mouse pups are exposed to 75% oxygen for 5 daysresulting in capillary obliteration in the central retina. Uponreturn to room air this central vasobliterated zone turns hypoxicand drives an angiogenic response. Currently we can only demonstratean angiogenic response 5 days later, at P17 when neovascularizationoccurs. In order to quantify the vascular response earlier,we quantified vascular tortuosity which is an early responseof arterioles to hypoxia.Our aim is to investigate a novel outcomemeasure of hypoxia in the mouse OIR model based on quantificationof vascular tortuosity at P14.

Methods: :
Pups from two mouse strains (C57Bl6 and C3H) were put throughthe OIR model and retinal wholemounts were retrieved at P14- 2 days after coming out of hyperoxia. The level of hypoxiawas assessed in both strains with a chemical hypoxia marker(EF5) and qPCR measurement of Vegf mRNA. Furthermore, wholemount retinas were stained with lectin and collagen IV (vesselmarkers). The tortuosity of radial arterioles was assessed indigital images using Image J software with a normalized measurementof the integrated curvature (nIC) and the tortuosity index (TI).

Conclusions: :
Measuring vascular tortuosity with our computer-based analysissystem of digital retinal images is an effective way of obtainingan indirect quantification of early hypoxia in the OIR model.This would be particularly useful in experiments testing cellularand molecular reversal of hypoxia and in experiments where anearly outcome measure of vascular pathology is desirable.